^aCenter for Marine Ecosystem Health, Harbor Branch Oceanographic Institute, Florida Atlantic University, 5600 U.S. I North, Fort Pierce, FL 34946, USA

Abstract

Coral reefs off southeast Florida have experienced an unprecedented succession of invasive chlorophyte blooms over the past two decades, most recently the non-native Caulerpa brachypus f. parvifolia. To better understand the ecology and nutrition of the C. brachypus invasion, we monitored benthic cover, water column dissolved inorganic nutrients, tissue C:N:P ratios and stable nitrogen isotopes (δ¹⁵N) of C. brachypus and native chlorophytes (Caulerpa racemosa, Caulerpa verticillata, Caulerpa mexicana, Codium isthmocladum) quarterly at two reef sites – the Princess Anne (PA) and North Colonel’s Ledge (NCL) – in 2003–2004. The PA site was influenced by stormwater discharges from the Lake Worth inlet whereas NCL was farther distant from these discharges. Between winter and spring of 2003, C. brachypus became the dominant benthic chlorophyte, expanding to >60% cover at both PA and NCL. Following cold temperatures (13 °C) associated with strong upwelling and high nitrate concentrations (21 μM) at NCL in July 2003, C. brachypus cover decreased, suggesting that upwelling can stress growth of this tropical alga. Mean ammonium (0.60 μM), nitrate (2.7 μM) and DIN (3.2 μM) concentrations were high for coral reef environments. Low mean C:N ratios of 13 in C. brachypus at both PA and NCL indicated little, if any, N-limitation compared to higher C:N ratios (up to 24) and greater N-limitation in native chlorophytes. Despite a relatively high mean SRP concentration (0.21 μM), mean N:P ratios of 39 in C. brachypus and other chlorophytes at PA and NCL suggested that these blooms were P-limited. Multiple lines of evidence support the hypothesis that land-based nutrient sources fueled the C. brachypus invasion. First, more persistent blooms of C. brachypus at PA compared to NCL correlated with significantly lower tissue C:P and higher δ¹⁵N values (wet season) at PA, the site most directly influenced by land-based stormwater runoff. Second, C:N, C:P, and δ¹⁵N values of C. brachypus correlated with seasonal patterns of rainfall and stormwater runoff. Third, δ¹⁵N values of C. brachypus and other chlorophytes decreased at NCL following strong upwelling in July 2003, confirming that upwelled nitrate was not the cause of the elevated δ¹⁵N values observed in these blooms. Lastly, the mean δ¹⁵N values of C. brachypus and other chlorophytes off southeast Florida (+4.9‰) were in the range of sewage nitrogen and significantly higher than values (+1.2‰) for reference chlorophytes in the Abacos, Bahamas, an area that experiences relatively little sewage input.